Field of the Invention
The present invention relates to an OCT apparatus for obtaining a tomogram by optical interference and a method of controlling the same.
Description of the Related Art
Various apparatuses are used at present as ophthalmic apparatuses using optical apparatuses. For example, various apparatuses such as a camera for anterior ocular segment, a fundus camera, and an SLO (Scanning Laser Ophthalmoscope) are used as optical apparatuses for observing an eye. In particular, an OCT (Optical Coherence Tomography) apparatus (to be referred to as an OCT apparatus hereinafter) is an apparatus for obtaining a tomogram of a sample at a high resolution, and is becoming indispensable as an ophthalmic apparatus in a specialty outpatient clinic for retina.
The OCT apparatus divides low-coherence light into reference light and observation light, irradiates a sample with the observation light, and uses interference between the reference light and reflected light from the sample, thereby forming an image in the observation light direction. The OCT apparatus scans the low-coherence light on the sample, thereby obtaining a tomogram of a section at a high resolution along the scan direction. For example, the OCT apparatus can capture a tomogram of the retina on the fundus of an eye to be inspected at a high resolution, and is widely used in ophthalmic diagnosis of retina and the like.
As a retinal tomogram obtained by the OCT apparatus, a scan called B scan is generally used. In the B scan, a scan in the retinal depth direction (Z direction) called A scan is performed a plurality of times while moving in the X direction, thereby obtaining a tomogram. The B scan allows a user to observe the internal state of the retina, as compared to an image of a conventional fundus camera or the like. It is therefore possible to obtain an image effective for observing a lesion in the retina such as macular degeneration or a macular hole in particular.
In imaging by the OCT apparatus, there exists a coherence gate that is a position where the reference light path and the observation light path have the same length. Japanese Patent Laid-Open Nos. 2008-154939 and 2008-154941 disclose a technique of adjusting an image position or signal-to-noise ratio (SN ratio) in an OCT image by adjusting the coherence gate position.
An OCT image has the origin at the coherence gate position and forms a tomogram by obtaining the values of the distance and reflection intensity. Hence, for example, when a retinal tomogram in a case where the coherence gate is located on the vitreous side with respect to the retina is defined as a normal image, a tomogram obtained in a case where the coherence gate is located on the choroid side with respect to the retina is a reverse image.
In addition, the reflection intensity (interference light intensity) of an image is the highest at the coherence gate position and lowers as the distance increases (called roll-off). For this reason, to obtain information of a retinal surface such as a nerve fiber layer concerning glaucoma, an OCT image is preferably obtained by placing the coherence gate on the upper retinal surface (vitreous side). On the other hand, to obtain information near a pigment epithelium layer concerning age-rated macular degeneration or the like, an OCT image is preferably obtained by placing the coherence gate on the lower retinal surface (choroid side). As described above, it is important in OCT imaging to determine whether the coherence gate is located on the vitreous side or choroid side with respect to the retina.